MLCCs are widely used in a variety of electronic applications and their use continues to expand. Of utmost importance for the instant discussion is their continued growth for use in alternating current (AC) applications. Even more important herein is their continued and growing use in AC applications with increasing amounts of AC voltage wherein the ripple current produced in the capacitor causes internal heating which can ultimately lead to failure.
Power (P) dissipation in an MLCC is defined by the equation:P=I2R wherein I is the current and R is the equivalent series resistance (ESR). Therefore, the heating increases as the square of the ripple current produced in the capacitors. There is also a frequency dependency and as the ESR declines, with increasing frequency, so does this self-heating. The ongoing desire to decrease ESR in MLCC's has mitigated the problem in the past. As the current increases further decreased ESR is no longer sufficient to mitigate the heat generation leading to the necessity of improved heat generation mitigation or removal of the heat being generated.
Heat is generally dissipated at the surface of the capacitor either at the surface of the ceramic or by conduction through the metal terminations. Since the internal temperature of a capacitor is difficult to determine it has been generally hypothesized that the surface temperature is a reasonable representation thereof. Based on this hypothesis a self-heating of 20-25° C. at the surface has been considered a safe condition for these types of capacitors with higher surface temperatures expected to result in thermal runaway and failure of the MLCC. The internal metal electrodes are effective heat conductors whereas the ceramic dielectrics are typically very good thermal insulators.
Increasing the number of internal electrodes can reduce ESR and therefore reduce self-heating. It has been an ongoing trend to increase the number of internal electrodes in an effort to increase the Capacitance, C. Capacitance is defined by the following equation:C=εrε0An/t where εr is the relative permittivity of the dielectric; ε0 is a constant equal to the permittivity of free space; A is the overlap area of two opposite polarity internal conductive layers separated by the dielectric, also referred to as an active; n is the number of actives and t is the separation distance or thickness between the electrodes. Therefore, the desire for higher capacitance has led to an increase in the number of layers and overlap area while decreasing the layer separation. However, in a given MLCC volume reducing the active thickness of the ceramic to increase capacitance further reduces the voltage handling capability of the MLCC although it allows more active layers and electrodes to be incorporated in the available volume. Increasing the number of electrodes is desirable since they conduct heat away but there is a compromise that has to be reached since voltage capability is reduced. Furthermore, any excess heat generated at the center of the MLCC becomes more difficult to remove and therefore the interior can be far hotter than the surface temperature would suggest and measuring the surface temperature as an indicator of the internal temperature becomes less reliable. Any minor differences in the capacitor construction resulting in thinner ceramic actives can result in high temperature spots or ‘hotspots’ that eventually fail under high AC power and the increased internal heat generation is difficult to detect.
The self-heating in an MLCC as a function of AC ripple current is shown graphically in FIG. 1. At a given frequency, an increase in current results in an increase in self-heating eventually leading to thermal runaway and failure of the MLCC. In addition, if the MLCC is at a high ambient temperature the self-heating can cause the rated temperature for the MLCC to be exceeded. Furthermore, surface heat is readily dissipated through the metal exterior terminations and surface of the MLCC through various techniques, such as the use of heat sinks and the like, but the surface temperature can be significantly exceeded by the internal temperature of the MLCC. Since the ceramic is a poor thermal conductor there is no efficient way to remove the heat from the interior of the capacitor, except by conduction through the internal electrodes, and this has proven to be insufficient at higher AC voltages.
There is an ongoing desire in the art for an MLCC which can withstand ever higher AC voltages without damage to the MLCC due to the increased self-heating. Provided herein is an MLCC which better dissipates heat from the interior of the capacitor body thereby mitigating the effects of self-heating.